A transmission power control method in a conventional mobile communication system will be explained. As shown in FIG. 1, the mobile communication system generally comprises a mobile station 1, radio base stations (to be simply referred to as base stations hereinafter) 2 and 3, and a base station control device 4 which controls these base stations.
Power control of the up link (UL) will be explained. The base station measures an SIR (Signaling Interference Ratio) equivalent to a UL radio reception level, and compares the SIR with a UL target SIR (target transmission power value) sent from the base station control device 4. If the measured SIR is lower than the target SIR, the base station instructs the mobile station 1 via the down link (DL) to increase the UL transmission power by a step designated in advance. To the contrary, if the measured SIR is higher than the target SIR, the base station instructs the mobile station via the DL to decrease the UL transmission power by a step designated in advance.
The base station control device 4 receives UL radio qualities (including reception data qualities) sent from the base stations 2 and 3, and calculates information (target SIR) for controlling to keep the UL reception quality constant. The target SIR calculation method will be explained below. In the following description, the target SIR will be abbreviated to SIRt.
In order to maintain the BLER (BLock Error Ratio) of a reception data block at a constant value (target BLER: to be simply referred to as BLERt hereinafter), when the reception data block has an error, the SIRt is increased by ΔSIRt. That is, the SIRt is updated to (SIRt+ΔSIRt). If the reception data block does not have any error, the SIRt is decreased by ΔSIRt×BLERt/(1−BLERt). That is, the SIRt is updated to (SIRt−ΔSIRt×BLERt/(1−BLERt)).
More specifically, the SIRt serving as a target SIR is updated and controlled as shown in FIG. 9. A required SIR (dotted line in FIG. 9) is 5 dB, ΔSIRt is 0.5 dB, and the BLERt as a target BLER is 0.01 (1%). In this case, if the SIRt becomes lower than 5 dB, an error occurs in reception data, and the SIRt increases by ΔSIRt=0.5 dB. The SIRt is equal to or higher than 5 dB during (1−BLERt)/BLERt=99 (corresponding to 99 blocks), and becomes lower than 5 dB again upon the lapse of a time corresponding to the 99 blocks (in FIG. 9, the SIRt seems to be on the 5-dB dotted line, but in practice, is slightly below the 5-dB dotted line).
Then, an error occurs in reception data, the SIRt becomes higher again by ΔSIRt=0.5 dB, and the same operation is repeated. Transmission power control is executed so that the radio quality of the up link (UL) satisfies a target BLER (BLERt). The above-described transmission power control method is disclosed in, e.g., Ashwin Sampath et al., “On Setting Reverse Link Target SIR in a CDMA System”, Vehicular Technology Conference, 1997 IEEE 47th, Vol. 2, 4-7 May, 1997, pp. 929-933.
The transmission power control method in the conventional mobile communication system suffers the following problems. That is, when a required reception level for obtaining a required radio quality drops upon a change in radio propagation environment, the reception level cannot be quickly decreased following the drop of the required reception level.
In general, as the moving speed of a mobile station changes, the required SIR also changes. For example, when the mobile station changes from a high-speed moving state to a low-speed moving state within a short time, the required SIR changes from a high level to a low level. When the required SIR changes from 15 dB to 5 dB, as represented by a dotted line in FIG. 10, the above-described conventional method gradually decreases the target SIR (SIRt), as represented by a solid line in FIG. 10, and cannot quickly decrease the reception level. While the reception level drops, communication is done at excessive power with an excessive communication quality, wasting power consumption.